The signals picked up by seismic sensors (geophones or hydrophones) or by seismic receivers consisting of several interconnected sensors may be transmitted to a recording station in an analog form through conducting lines or cables. When the transmission distances are relatively long, acquisition devices located close to the sensors or to the receivers are generally preferably used which sample and digitize the signals received, and store and transmit the signals to a central station through specific transmission channels or transmission channels common to several acquisition devices. The digital words produced are coded for the transmission thereof. A bipolar code is used allowing both the data and a clock signal (A.M.I., HDB3, Manchester type codes, etc) to be transmitted. The clock and data are integrated into a transmission frame associated with identification, error detection or synchronization prefixes or suffixes. For these conversions, analog-to-digital converters of the over-sampling type, including the combination of a modulator and of a digital filter, may be notably used. The modulator is of the delta-sigma type and generates a series of low-resolution digital words in the form of a continuous bitstream at a very high sampling frequency (256 kHz for example). These words have a mean amplitude varying in time proportionally to the amplitude of the analog signals applied. Typically, these words comprise only one bit. The digital filter is connected to the modulator output. It receives therefrom a bitstream and produces high-resolution digital words (16 to 24 bits for example) of a much lower frequency in an operation known as decimation. Converters of this type are for example described in U.S. Pat. Nos. 4,866,442; 4,943,807; 4,994,804 or in French Patent Application FR-90/11,527 (corresponding to U.S. Pat. No. 5,245,647).
Conversion of analog signals into digital words may in some cases be achieved locally, in the immediate vicinity of the sensors or the receivers. U.S. Pat. No. 5,051,799 describes for example a transmission device where analog signals are applied to a local converter of the oversampling type including a modulator associated with a decimating digital filter and where the digital words coming from the converter are emitted on a transmission channel, included in a transmission frame with addition of error correction bits.
Whatever the type of converter used, a local module for acquiring signals, formatting the digital words produced by analog-to-digital conversion and transmitting each frame is costly and its power consumption is high.